1. Field of the Invention
The present invention relates to a two-component developing agent for electrophotography.
2. Description of the Related Art
An electrophotographic technique applying a Carlson process has widely been used in an image forming method using a developing agent. The image forming adopting the Carlson process is performed by a charging step, an exposing step, a developing step, a transferring step, a fixing step a cleaning step, a charge-removing step and the like. In the charging step, a surface of a photoreceptor is uniformly charged. In the exposing step, an electrostatic latent image is formed on the surface of the photoreceptor by exposing the thus-charged photoreceptor. In the developing step, a visible image is formed by allowing a developing agent such as a toner to adhere to the electrostatic latent image formed on the surface of the photoreceptor. In the transferring step, a toner image is transferred onto a recording material by charging the recording material with a polarity opposite to that of the toner. In the fixing step, the visible image transferred onto the recording material is fixed by applying measures such as heat and pressure. In the cleaning step, the toner left over on the surface of the photoreceptor without having been transferred to the recording material is recovered. In the charge-removing step, the charge on the photoreceptor is removed. By performing these steps, an image forming apparatus making use of an electrophotographic process forms a desired image on the recording material. A developing system based on the electrophotographic technique is roughly divided into a one-component developing system and a two-component developing system.
The one-component developing system is a system in which a layer made only of the toner is formed on a surface of a developing roller, the thus-formed layer is allowed to be disposed adjacent to the surface of the photoreceptor and, then, development is performed.
The two-component developing system is a system in which magnetic particles denoted as a carrier and the toner which have been friction-charged by mixing therewith are formed as a developing agent layer on the surface of the developing roller that holds a magnet inside and, then, the development is performed by allowing the toner to electrostatically adhere to the photoreceptor. Although an apparatus of the two-component developing system is somewhat complicated compared with that of the one-component developing system, since the two-component developing system is easy to set a potential of the toner and excellent in a property of responding to a high-speed operation and stability, the two-component developing system is mainly used in a medium- to high-speed printer. In the two-component developing system, a two-component developing agent constituted by the toner and the carrier is used.
The toner to be used in the two-component developing agent can be obtained by, for example, a crushing method in which a binder resin, an coloring agent, a charge control agent and a wax as an anti-offset agent are melt-kneaded, cooled to be solidified, crushed and, then, classified, or a polymerization method such as a dispersion polymerization method or an emulsion polymerization method. The thus-obtained toner is mixed with the carrier, to thereby prepare the two-component developing agent.
The carrier, which is constituted by particles having magnetism each denoted as a core material, and a resin coating layer to be formed on a surface of the core material, bears a function of stably charging the toner in a developing apparatus and transporting the toner to a developing region. The core material determines an amount of the carrier to adhere to the developing roller which holds the magnet inside in accordance with magnetic properties which the core material itself has. Further, the resin coating layer mainly bears a function of imparting the charge to the toner and determines a state in which the toner adheres to the carrier. In the two-component developing system which develops the electrostatic latent image by making use of an electrostatic attraction, in order to obtain a favorable visible image, it is necessary that a frictional chargeability of the toner to be determined mainly based on a relation with the carrier is favorable. Therefore, designs of the core material of the carrier and the resin coating layer to be formed on the surface of the core material and setting of a coating amount come to be important.
In recent years, a higher speed and a smaller size of each of a copying machine and a printer has been attempted. In order to stably obtain a high-quality image for a long period of time, durability and environmental stability of the developing agent itself has been required.
As for the two-component developing agent which is excellent in both durability and stability, one two-component developing agent in which a coating layer is contained in an amount, based on 100 parts by weight of the core material of the carrier, of from 0.1 to 5.0 parts by weight and the coating layer is an organic resin having a nitrogen-containing compound is proposed (for example, refer to Japanese Unexamined Patent Publication JP-A 4-177369 (1992)).
However, when this developing agent is used in, for example, a copying machine which has realized a smaller size and a higher operation speed, the coating layer of the carrier is peeled off in use over a long period of time, to thereby expose a surface of the core material which is intrinsically weak. As a result, there is a problem in that initial carrier characteristics have been changed to a great extent to cause a deterioration of an image quality.
Further, in recent years, a two-component developing agent which is excellent in durability and stability by adding a metallic oxide such as titanium oxide into a resin of a coating layer of a carrier is proposed (for example, refer to Japanese Unexamined Patent Publication JP-A 2003-66656). However, since this developing agent is characterized by allowing the metallic oxide contained in the resin of the coating layer to have a density gradient, a production process comes to be extremely complicated. Therefore, there is a problem in that it is difficult to perform a mass-production by such technique as described above and, then, a low-priced product cannot be supplied to customers in a consistent manner.
Further, a toner tends to have a small particle size to meet demand of a higher image quality. When a specific surface area of the toner is increased in accordance with a decrease of the particle size, frictional charge comes to be large, to thereby increase an adhering force between toners themselves and between the toner and the carrier. As a result, fluidity of the developing agent is decreased and, then, since the toner is not stably supplied to the photoreceptor, there is a problem in that an image density is decreased. On the other hand, when a stirring force in a developing apparatus is increased for the purpose of enhancing the fluidity of the developing agent, a endurance life of the developing agent is shortened. Further, when a potential of the toner is decreased for the purpose of preventing the image density from being decreased, an adhering force between the toner and the carrier is decreased, to thereby allow the adhering force between the toners themselves to be over the former adhering force. As a result, toners are aggregated with each other and, then, flowed onto the photoreceptor as an aggregated block and, accordingly, although the image density is increased, the image quality is deteriorated and, also, the toner is scattered, to thereby cause a problem in that an inside of the apparatus is contaminated. Under these circumstances, a two-component developing agent which can balance these contradictory properties and exhibit only an advantage is strongly required.